The present invention relates to a method for securing selected portions of heat-sealable wrappings applied to products.
In particular, the present invention relates to a method of securing the end folds of overwrappings fashioned in transparent heat-sealable material around packets of cigarettes, to which reference is made explicitly in the following specification, albeit implying no limitation in the general scope of the disclosure.
Overwraps of the type in question are produced normally with a "cellophaner", a machine comprising a unit by which a sheet of the transparent material in question is folded around a packet to form a tubular wrapper of which the opposite ends project initially beyond the end faces of the packet; each projecting end is then folded inwards to form two partially overlapping flaps, which are flattened into contact with the relative end face and united by heat-sealing. It is the conventional practice to secure these overlapping flaps using a heat-seal device that comprises two identical belts looped around respective pulleys; such belts are generally metallic and heated electrically. The two belts are arranged with two respective rectilinear branches extending mutually parallel and advancing in the same direction, in such a way that a feed channel is created between them.
Given that each end face of the cigarette packet reacts to externally imposed mechanical stresses by inflecting, as if it were a plate restrained about the periphery, the operating surfaces of the two belts are furnished with respective pluralities of uniformly distributed projections or pads caused to advance synchronously along the feed channel. As the packets pass along the feed channel between the two belts, pressure is applied by the pads to the overlapping flaps and the end faces of the packets in the appropriate manner, with the result that the flaps are sealed together by the combined action of pressure and heat.
The pads are heated by conduction through contact with the metal conveyor belt, the belt in its turn being heated electrically.
In one particular heat-seal device of this type, the belt and the relative sets of pads are heated by electromagnetic induction. Current generated by an expander device linked to the belt is caused to circulate by way of electrical conductors extending along the full length of the belt and connecting the pads one with another in short circuit.
The electrical resistance of the pads is higher than that of the conductors embedded in the belt, so that with an induced current circulating through the conductors, the temperature of the pads is raised to a value higher than that of the belt by the resulting Joule effect.
The main drawback encountered with solutions of the type described above is attributable to the manner of heating the pads.
To heat the entire metallic conveyor belt so as to heat the pads, or to exploit the effect of mutual induction generated between the belt and the expander to circulate a current along the full length of the belt, it becomes necessary to heat a relatively large mass of material, which involves a waste of heat, and therefore of energy, but more especially brings about a situation typified by significantly high thermal inertias. As a result, the temperature of the pads cannot be regulated quickly to adapt to the instantaneous velocity of the machine. Given the proportions of the mass to be heated, moreover, one has a rise in temperature affecting the entire area around the conveyor belt, and consequently an undesirable transfer of heat to the packet as a whole during the operation by which the end folds are secured.
The object of the present invention is to provide a method whereby selected portions of heat-sealable product wrappings can be secured without any of the drawbacks mentioned above in connection with the prior art.